Recent Developments in Heterogeneous Catalysis of N-Methylation Reaction with CO₂

Abstract

A promising strategy for reducing CO₂ emissions is to catalyze the conversion of CO₂ into high-value products, such as N-methylamines, which serve as key intermediates in various industries including pharmaceuticals, agrochemicals, and fine chemicals. This review summarizes recent advancements in the application of heterogeneous catalysts for the N-methylation of amines via CO₂ hydrogenation. Various types of catalysts are discussed, including noble metal-based catalysts (e.g., Pd, Ag, Au, Pt, and Ru), non-noble metal catalysts (e.g., Cu and Zn), and non-metallic catalysts (e.g., nitrogen-doped carbon materials). The review analyzes catalyst performance, reaction conditions, and reaction mechanisms, with a focus on the relationship between catalytic activity, product yield, reaction mechanisms and support, metal, modifier, and so on in the catalytic process.

 

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Keywords

CO₂ - N-methylation - Hydrogenation - Heterogeneous catalysis

Significance

A promising strategy for reducing CO₂ emissions is to catalyze the conversion of CO₂ into high-value products. CO₂ as a renewable C1 source could be used as a methyl reagent in the N-methylation of amines. The main objective of this review is to extract information from existing literature and provide information on how to overcome the major challenges associated with N-methylation with CO₂ by carefully designing catalyst systems with appropriate metals and supports, as well as reaction conditions, to activate CO₂ with high activity and selectively produce N-methyl or N,N-dimethyl products.

Introduction

In the past 50 years, human activities have led to excessive CO₂ emissions, raising the concentration of CO₂ in the atmosphere from 320 to 426 ppm. The large-scale release of CO₂ has become an increasingly severe global issue. In 2024, approximately 42 billion tons of CO₂ was emitted into the atmosphere. The increase in CO₂ levels in the atmosphere exacerbates the greenhouse effect and ocean acidification, both of which have profound impact on global climate and ecosystems. Therefore, there is an urgent need to develop new technologies for reducing CO₂ emissions. Currently, some solutions are being actively explored and optimized, both in scientific research and engineering applications. Carbon capture and storage is becoming a promising technology for reducing emissions, and several demonstration power plants have been put into operation. However, challenges still exist, particularly the high energy and economic costs associated with CO₂ capture, deposition, and storage, which have not been fully addressed. Against this backdrop, the utilization of CO₂ to synthesize high-value chemical products or fuel offers an attractive approach for CO₂ utilization while simultaneously reducing emissions and adding economic value to the process of CO₂ capture and utilization.

In the field of CO₂ utilization, converting CO₂ as a carbon resource into fuels (e.g., carbon monoxide [1], [2], methanol [3], [4], formic acid [5], [6]), chemicals (e.g., amides [7], [8], methylated products [9], [10]), and materials (e.g., carbon nanotubes [11], [12], carbon fibers [13]) has emerged as a promising approach. These strategies have attracted widespread attention from both academia and industry as CO₂ not only is consumed but can also generate valuable products, effectively transforming waste into wealth. However, CO₂ conversion faces substantial challenges due to the CO₂ molecule’s strong thermodynamic stability and kinetic inertness. The carbon atom in CO₂ exists in its highest oxidation state, and the molecule as a whole resides in a low-energy state, making it particularly difficult to activate and subsequently convert into other carbon-containing compounds. At present, catalytic CO₂ conversion has become a global research hotspot although there are still many problems in the catalytic process, including poor selectivity, numerous side reactions, and low activity. The development of highly active catalysts, alongside the exploration of novel catalytic methods, is critical for achieving high-value utilization of CO₂, and these endeavors will have profound significance.

N-Methylation is a critically important reaction in organic chemistry. CO₂ as a renewable C1 source could be used as a methyl reagent to replace HCHO, HCOOH, MeI, and CO in the N-methylation of amines [14] [15] [16] [17]. Under catalytic conditions, CO₂ can be reduced by H₂ or organosilanes, causing the N-methylation or N-formylation of amines. N-alkylamines have extensive applications and serve as key intermediates in various industries, including petrochemicals, the rubber industry, dye manufacturing, pharmaceutical development, and the synthesis of fine chemicals. For example, N,N-dimethylaniline plays an essential role in agricultural chemistry, where it can be used to synthesize useful herbicides and also serves as a critical intermediate for basic dyes. Moreover, in the fragrance industry, N,N-dimethylaniline is employed in the synthesis of vanillin and other aromatic compounds.

Designing efficient catalysts for the hydrogenation of CO₂ and the subsequent N-methylation reaction of amines is crucial. In recent years, as the synthesis of N-methylation production via N-methylation with CO₂ has been comprehensively explored, there have been few reviews published [10], [18] [19] [20] [21]. Compared with these reviews, the present review aims to provide an overview of the catalytic effects of various precious and non-precious metals on different supports in the N-methylation reaction. In addition, this review delves into the key role of active species generated by metals or their oxides as well as the role of supports. Besides, the influencing factors of the reaction, including solvents and reaction conditions, have also been discussed. Moreover, the mechanism of CO₂ activation and the subsequent N-methylation effect were compared and discussed over various catalysts, including the same metal on different supports and different metals on the same support. All in all, the main objective of this review is to extract information from existing literature and provide information on how to overcome the major challenges associated with N-methylation with CO₂ by carefully designing catalyst systems with appropriate metals and supports, as well as reaction conditions, to activate CO₂ with high activity and selectively produce N-methyl or N,N-dimethyl products.

 

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Results and Discussion

Catalyst in N-methylation of amines with CO₂ and H₂

In the field of CO₂ hydrogenation, H₂ is a widely employed reducing agent due to its cost-effectiveness and sustainability. H₂ could be produced through electrolysis of water, while electricity could be generated by using renewable energy sources (such as solar, wind, and hydro power), the production process of H₂ has no carbon emissions and conforms to the concept of low-carbon environmental protection. When H₂ is used as a reductant, the reaction exhibits excellent atom economy, with water being the only non-toxic and harmless byproduct. This makes the process not only environmentally friendly but also economically viable, presenting a green and sustainable solution with minimal impact on the environment.

Cu-based catalyst

In 1995, Baiker’s group first reported the N-methylation of NH₃ with CO₂ and H₂ using a Cu/Al₂O₃ catalyst [22]. They observed that varying the concentration of NH₃ would result in different products; at low NH₃ concentrations, trimethylamine was predominantly formed, while high NH₃ concentrations favored the formation of methylamine. The same group continued their investigations and found that the higher the reaction temperatures, the higher the selectivity to methylamine [23]. They further explored the effect of different catalyst supports and found that Cu catalysts supported on supports with a point of zero charge between 6 and 8 (such as Cr₂O₃, ZrO₂, and Al₂O₃) exhibited higher activity [24]. Subsequently, they prepared catalysts loaded with different metals on Al₂O₃ and found that only Cu produced methylamine as the main product, while other metals like Ni, Co, Pt, Fe, and Ag mainly yielded CO or methane [25]. In 1999, the same group synthesized a Cu–Mg–Al ternary catalyst via co-precipitation for the N-methylation of NH₃ with CO₂ and H₂ [26]. They studied the effect of the composition and calcination temperature, discovering that the highest catalytic activity was achieved when the hydrotalcite phase was the sole phase present in the catalyst. The highest catalytic activity was observed when the catalyst was calcined at 500 °C. Under reaction conditions of 280 °C, 0.12 MPa CO₂, and 0.36 MPa H₂, the amine formation rate of the catalyst is 0.83 mol kg⁻¹ cat⁻¹, and the selectivity to methylamine is as high as 86%. However, when the calcination temperature was raised to 600 °C, the appearance of CuAl₂O₄ phase led to a reduction in catalytic activity.

In 2014, Shi’s group developed a CuAlO _x catalyst via co-precipitation by adding Na₂CO₃ as a precipitating agent to a mixed solution of Cu(NO₃)₂ and Al(NO₃)₃. This catalyst was applied to the N-methylation reactions of primary amines, secondary amines, and nitrobenzenes, demonstrating high selectivity in converting substrates into N-methyl or N,N-dimethyl products. In the N-methylation of aniline, the CuAlO _x catalyst achieved an 86% yield of monomethylated N-methylaniline (MA) under conditions of 160 °C, 3 MPa CO₂, and 6 MPa H₂ in 24 h. For the further N-methylation, an 83% yield to the dimethylated product of N,N-dimethylaniline (DMA) could be obtained by increasing the H₂ pressure to 7 MPa and extending the reaction time to 48 h [27].

In 2017, Tomishige’s group prepared a Cu/CeO₂ catalyst via the impregnation method. The catalyst was tested in the N-methylation of aniline at 170 °C under 1 MPa CO₂ and 7 MPa H₂; a conversion of 28% with 95% selectivity to MA could be obtained in 4 h. A conversion of 70% and a selectivity of 99% to MA could be obtained by extending the reaction time to 42 h. However, by further prolonging the reaction time, the conversion of aniline did not significantly increase, while the selectivity to MA decreased due to the further N-methylation of MA to DMA. As the amount of water added increased from 0 to 4 mmol, the conversion decreased from 22% to 4.4%, indicating that the water produced during the N-methylation process as a byproduct would poison Cu/CeO₂. Further studies on catalysts with different Cu loadings showed that larger Cu particles were unfavorable for the reaction, while sub-nanometer Cu particles were the main active species. They also investigated the effect of CO₂ and H₂ pressures. The CO₂ pressure had little effect on conversion and selectivity, while both conversion and selectivity increased with H₂ pressure [28].

In 2019, our group developed a Cu/TiO₂ catalyst via a deposition-precipitation method for the N-methylation of MA to DMA. When CeO₂, ZnO, and C were used as supports, both the conversion and DMA selectivity were lower than those when TiO₂ was used as a support. A conversion of 82% and a selectivity of 98% to DMA were obtained over Cu/TiO₂ under reaction conditions of 180 °C, 2 MPa CO₂, and 4 MPa H₂ in 36 h. The conversion of MA increased linearly with the amount of the exposed Cu⁺ and Cu⁰ species ([Figure 1]); therefore, the surface Cu⁺ and Cu⁰ species exposed on the catalysts should be the active sites. Further investigation into the reaction pathway suggested that CO₂ is first hydrogenated to form the intermediate CHO*, which then reacts with MA to form N-methylformanilide (MFA). Finally, MFA undergoes hydrogenation and dehydration to yield DMA ([Scheme 1]). In recycling tests, the catalyst showed a significant drop in activity in the third and fourth cycles. After cycling, the average particle size of Cu nanoparticles increased from 2.3 to 3.6 nm, and carbon deposition occurred on the Cu/TiO₂, resulting in the catalyst deactivation. However, the activity of the catalyst can be recovered after calcination at 450 °C for 4 h, followed by reduction at 250 °C for 2 h [29].

Figure 1 The relation between total conversion and the amount of Cu⁺ and Cu⁰ species exposed on the surface of the catalyst. Reprinted with permission from Ref. [29]. Copyright 2019 Wiley. [rerif]

Scheme 1 The reaction pathways for N-methylation of MA with CO₂ and H₂ over 5% Cu/TiO₂ catalyst. Reprinted with permission from Ref. [29]. Copyright 2019 Wiley. [rerif]

In 2024, He’s group prepared supported Cu catalysts via the impregnation method for the N-methylation of amines. Catalytic performance of various In₂O₃ supported metal catalysts (Cu, Fe, Co, Ni, and Pd) and other supports (TiO₂, CeO₂, and SiO₂) that supported Cu was compared in N-methylation of MA with CO₂ and H₂ under the conditions of 180 °C, 1 MPa CO₂, and 5 MPa H₂. Without supporting any other metal, the catalytic performance of In₂O₃ was poor, and only a 10% yield of DMA could be achieved. Among the In₂O₃-supported metal catalysts, Cu/In₂O₃ showed the highest conversion of MA. As the loading of Cu increased from 0% to 2%, the conversion increased from 18.1% to 85%, and the selectivity increased from 55.5% to 100% ([Scheme 2]). Ether-based solvents, such as THF and DME, provide relatively low conversion, and almost no reaction occurs in alcohol; these results suggest that the oxygen in the solvent partially or completely blocks the existing oxygen vacancies of the catalyst, which may prevent the interaction between the substrate and the catalyst. Among the tested solvents, octane is the best one. The effects of reaction temperature on the catalytic performance have also been investigated. Both the conversion and selectivity were enhanced progressively with the increase in temperature, probably because the higher temperature favors the activation of CO₂ and H₂. The Cu⁺ was the main species on the surface of catalysts before and after reaction; thus, the Cu⁺ species should be the main active sites. CO₂ was activated with H₂ to form formate on the surface of the catalyst, and then the formed formate was reacted with MA to form MFA; finally, MFA can be quickly hydrogenated to the final product of DMA. The difference in the catalytic performance may be related to the difference in the adsorption of MFA on In₂O₃ and Cu/In₂O₃. The strong adsorption through C=O of MFA on Cu/In₂O₃ resulted in MFA being highly selectively hydrogenated to DMA over Cu/In₂O₃. But the fast C–N bond cleavage of MFA back to MA and the slow hydrogenation of MFA to DMA resulted in poor catalytic performance of In₂O₃ [30].

Scheme 2 N-Methylation of amines with CO₂ and H₂ over In₂O₃ and Cu/In₂O₃. Reprinted with permission from Ref. [30]. Copyright 2024 Wiley. [rerif]

In 2024, Yuan’s group prepared a Cu–ZrO _x /SBA-15 catalyst via oxalate precipitation for the N-methylation of aniline to MA using CO₂ and H₂. The dispersion of Cu–ZrO _x on the surface of SBA-15 is superior to the other two molecular sieves (silicalite-1 (S-1) and ZSM-5) and effectively prevents the sintering of active metals. The conversion of aniline and the selectivity to MA over Cu–ZrO _x /SBA-15 are much higher than that over Cu–ZrO _x /S-1 and Cu–ZrO _x /ZSM-5. The dispersion of Cu might not be the only factor affecting catalytic performance, and the acidity–basicity properties of the catalyst may also play a crucial role. Among the Cu–ZrOx/SBA-15 catalysts with different Cu loadings, the catalyst with a loading of 20 wt % has the best catalytic activity, it exhibited a conversion of 97.6% with a selectivity of 96.5% for MA under the reaction conditions of 4  MPa total pressure (H₂/CO₂/N₂ = 72/24/4) and 180 °C in 8 h. The control experiments with C1 compounds seem to indicate that methanol, formic acid, and formaldehyde were not intermediates in the reaction. Aniline preferentially adsorbs on the acidic sites of the catalyst surface. CO₂ was carbonylated with the adsorbed AN, generating phenylcarbamic acid, and it underwent hydrogenation to form formanilide and further hydrogenation of formanilide formed MA ([Scheme 3]). Compared to the hydrogenation of CO₂ to form C1 intermediates, the direct C–N coupling between the amine and CO₂ helps prevent the undesired self-coupling of amines [31].

Scheme 3 Proposed reaction mechanism for N-methylation of AN using CO₂/H₂ over Cu–ZrO _x /SBA-15. Reprinted with permission from Ref. [31]. Copyright 2024 Elsevier. [rerif]

 

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Pd-based catalyst

As early as 1997, Baiker’s group prepared Pd/Al₂O₃ for the N-methylation of NH₃. They discovered that NH₃ and methylamine adsorbed onto the Brønsted and Lewis acid sites of the catalyst favor the N-methylation of NH₃ to methylamine [32]. In 2014, Shi’s group developed a PdCuZrO _x catalyst via precipitation for the direct N-methylation of N-methylpiperazine using CO₂ and H₂. The catalyst achieved a 97% yield under relatively mild conditions (1 MPa CO₂, 2.5 MPa H₂, 150 °C, 30 h). Upon expanding the substrate scope, the catalyst was found to be effective for both aliphatic and aromatic amines, demonstrating a degree of versatility. The PdCuZrO _x catalyst primarily produced 97% of N-methylated products in 30 h. However, when the PdZnZrO _x catalyst was used in 15 h, the N-formylation reaction became dominant [33].

In 2016, our group prepared a PdGa/TiO₂ catalyst via precipitation. The catalyst exhibited a 98% conversion and 94% selectivity in the direct N-methylation of MA to DMA under reaction conditions of 5 MPa CO₂, 5 MPa H₂, and 180 °C in 10 h. The PdGa alloy was highly dispersed within the TiO₂ matrix, with electron transfer occurring from Pd to Ga. This electron-deficient Pd, in interaction with Ga, enhanced the ability to activate CO₂. The electron-deficient Pd in the PdGa alloy activates CO₂ to form HCOO*, which then reacts with MA to produce MFA. Subsequently, this intermediate of MFA undergoes hydrogenation to DMA [34]. In 2020, our group synthesized a series of Pd-ZnO/TiO₂, Pd/TiO₂, and Pd/ZnO catalysts via precipitation and investigated for N-methylation of MA to DMA with CO₂ and H₂. A much higher performance was observed with a Pd-ZnO/TiO₂ catalyst compared with Pd/TiO₂ and Pd/ZnO; a high selectivity to DMA (99.9%) at the MA conversion of 94% could be obtained. The catalytic performance of Pd-ZnO/TiO₂ largely depended on reduction temperature and ZnO loading. The rates for MA conversion and DMA production increased linearly with the amount of PdZn alloy ([Figure 2]). The PdZn alloy was the active species and played a crucial role in the reaction. The PdZn alloy formed is significantly effective in the activation of CO₂ to produce formate, in the N-methylation of MA with formate to MFA, in the hydrogenation of MFA to DMA, and in the prohibition of C−N bond cleavage side reactions ([Scheme 4]), thereby the conversion of MA and the selectivity to DMA enhanced with the increase in the amount of the surface PdZn alloy [35].

Figure 2 (I) DMA selectivity (circle open) as a function of the ratio of M_PdZn to M_Pd+PdZn and (II) rate_MA (circle solid) and rate_DMA (triangle up open) as a function of PdZn alloy content (M_PdZn). (a) Pd/TiO₂-400, (b) Pd-ZnO(7.5%)/TiO₂-100, (c) Pd-ZnO(7.5%)/TiO₂-400, (d) Pd/ZnO400, (e) Pd/TiO₂+ZnO(7.5%)-100, (f) Pd/TiO₂+ZnO(7.5%)-400, and (g) Pd-ZnO(1%)/TiO₂-400. Adapted with permission from Ref. [35]. Copyright 2020 American Chemical Society. [rerif]

Scheme 4 Reaction pathways for (A) PdZn and (B) Pd-catalyzed N-Methylation of MA with CO₂ and H₂. Reprinted with permission from Ref. [35]. Copyright 2020 American Chemical Society. [rerif]

At present, our group has prepared a kind of bimetallic PdIn/TiO₂ catalyst and studied the N-methylation reaction of amines with CO₂/H₂, achieving a high yield of up to 99%. The relationship of TOF for DMA was positively correlated with the amount of the surface-exposed PdIn alloy, which confirmed that PdIn alloy was the main active species for converting CO₂ to a *HCOO species. A possible catalytic mechanism for the N-methylation of MA with CO₂ and H₂ is proposed and shown in [Scheme 5]. For PdIn/TiO₂, CO₂ was adsorbed and activated on the surface oxygen defects of oxide that were close to PdIn alloy; H₂ was activated on the surface of PdIn alloy to active H^δ– species, as demonstrated by the reaction kinetics and supported by DFT theoretical calculations. The strong nucleophilic H^δ– was inclined to attack the positive carbon of CO₂, forming *HCOO. Then, the reaction of MA and *HCOO species occurred quickly with MFA. Finally, MFA was hydrogenated and dehydrated to form DMA. The formation of H^δ– over PdIn alloy was favorable to the hydrogenation of CO₂ to *HCOO and MFA to DMA, inhibited the C–N cleavage, and inhibited the hydrogenation of the aromatic rings. Thus, the selectivity to DMA was much higher over PdxIn/TiO₂ catalysts. For the Pd/TiO₂ catalyst, H₂ was activated on the surface of Pd species and dissociated to active H* species; the H* active species formed on Pd/TiO₂ induced the hydrogenation of CO₂ to *CO and *HCO species but not to *HCOO species. Even if *HCOO could be slightly generated, *HCOO reacted with MA to form MFA spontaneously, but MFA mainly underwent C–N cleavage and aromatic ring hydrogenation on Pd/TiO₂. Thus, the formation of DMA was difficult over Pd/TiO₂ [36].

Scheme 5 The possible reaction mechanism for CO₂ hydrogenation to *HCOO/ HCOOH species for the N-methylation of MA over Pd/TiO₂ and PdIn/TiO₂. Reprinted with permission from Ref. [36]. Copyright 2024 Elsevier. [rerif]

 

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Au-based catalyst

In 2015, Wang’s group prepared an Au supported on Al₂O₃ catalyst via precipitation, where the Au nanoparticles had a very small size, approximately 2 nm (named as Au/Al₂O₃-VS). The catalytic activity and product yield over Au/Al₂O₃-VS were much higher than that over the Au/Al₂O₃ catalyst supplied by Mintek, which has a larger average Au particle size (~3.0 nm). Both the noble metal and the support play important roles in the N-methylation of aniline to DMA with CO₂ and H₂. By changing the support, Au/Al₂O₃-VS was also more effective than Au/TiO₂, Au/ZrO₂, Au/CeO₂, Au/ZnO, and Au/SiO₂. By changing the metal, Au/Al₂O₃-VS was also more effective than Pt/Al₂O₃, Ir/Al₂O₃, and Rh/Al₂O₃. A 92% yield of DMA in the N-methylation of aniline could be obtained over Au/Al₂O₃-VS under reaction conditions of 2 MPa CO₂, 6 MPa H₂, and 140 °C in 7 h. Expanding the substrate scope revealed that anilines with electron-donating groups exhibited high reactivity, while anilines with electron-withdrawing groups showed relatively lower reactivity. However, the catalyst exhibited poor activity for some aliphatic amines and sterically hindered secondary amines [37]. In 2015, Zhong’s group also developed an Au/Al₂O₃ catalyst via precipitation for the N-methylation of MA. They found that solvent polarity had a significant impact, apolar solvents showing better reactivity compared to polar ones. A support with relatively strong acid-base dual functional sites may be necessary for efficient N-methylation of MA with CO₂ and H₂ because the basicity of the support facilitates CO₂ adsorption and accelerates product desorption from the catalyst surface, while the acidity enhances amine adsorption. Al₂O₃ with relatively strong acidic and basic sites exhibits the best catalytic performance. When TiO₂ and CeO₂ with a few acidic sites are used, the catalytic activity is moderate. Other supports, such as SiO₂ with few acidic sites, Mn₂O₃ and ZnO with weak acid–base properties, Co₃O₄ and C with neither acidic nor basic site, and MgO with relatively strong basic sites, have little or very low catalytic activity. Additionally, by varying process parameters such as aging time and temperature during catalyst preparation, they synthesized Au/Al₂O₃ catalysts with different particle sizes. They found that as the particle size decreased from 8.3 to 1.8 nm, the turnover frequency (TOF) increased, reaching a maximum value of 287 h⁻¹ at 1.8 nm, and the Au-catalyzed direct methylation of N-methylaniline with CO₂/H₂ was a structure-sensitive reaction [38].

In 2020, Ji’s group synthesized a metal-organic framework (MOF-808(Zr)) with unsaturated Zr⁴⁺ coordination centers (CUS) using a thermal reflux method to study the impact of support acidity and basicity on the reaction. Au was then loaded onto the MOF-808(Zr) via impregnation. This catalyst was applied to the N-methylation reactions of primary and secondary amines, revealing that primary amines more readily produced N-methylated products, while secondary amines favored N-formylation. This difference was likely due to steric hindrance and electronic effects. H₂ is adsorbed and activated on the Au nanoparticles. CO₂ is adsorbed onto the acidic Zr⁴⁺ and basic O²⁻ sites, and then reacts with activated H₂ to form formic acid. Amines are adsorbed onto the CUS-Zr⁴⁺ acidic sites, where they react with formic acid to produce N-formylated products. These N-formylated products subsequently react with activated H₂ to form N-methylated products [39].

 

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Pt-based catalyst

In 2014, Shimizu’s group prepared a Pt-MoO _x /TiO₂ catalyst via a stepwise impregnation method. This catalyst was used in the N-methylation of MA under solvent-free conditions, achieving 85% DMA yield under the reaction conditions of 1 MPa CO₂, 4 MPa H₂, 200 °C in 24 h [40]. For supported Pt catalysts, Pt/Nb₂O₅ showed a higher yield of the desired product. Pt/C, as a conventional Pt catalyst, exhibited a high conversion (100%) but resulted in a low yield of the product, partly due to aromatic ring reduction. Although the conversion (7%) and yield (6%) were low over Pt/TiO₂, the catalytic performance was significantly improved after modification with oxides of transition metals (V, Mo, W, Re), with Pt-MoO _x /TiO₂ showing the best performance (Conversion 85%, Yield 85%). For the co-loading Pt and MoO _x , the support has an important role; TiO₂ was much better than Al₂O₃, ZrO₂, SiO₂, and Nb₂O₅. A series of transition metal and MoO _x coloaded on TiO₂ was tested. Among various metals (Pt, Pd, Rh, Ru, Ni, Cu), Pt-MoO _x /TiO₂ showed the highest yield. These results indicate that the co-presence of Pt and Mo species on TiO₂ is critical. In this system, formic acid is formed by Pt-catalyzed hydrogenation of CO₂, which reacts with amine to give MFA, which undergoes Pt-catalyzed hydrogenation to give tertiary amine [40]. In 2017, the same group employed Pt-MoO _x /TiO₂ to directly react NH₃ with CO₂ and H₂ to produce trimethylamine, achieving a 64% yield under reaction conditions of 1 MPa CO₂, 5 MPa H₂, and 250 °C in 24 h [41]. In 2017, the same group developed a Re/TiO₂ catalyst, which exhibited excellent catalytic performance in the N-methylation of amines with CO₂ and H₂ [42].

 

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Other catalysts

In 2023, Zhu’s group developed an Ag/Al₂O₃ catalyst, which selectively catalyzed the mono-N-methylation of aniline to MA while suppressing the further N-methylation of MA to DMA. When cyclohexane was used as a solvent, a 31% conversion of aniline with excellent selectivity (>99%) of the target product was achieved. Hexadecane, on the other hand, showed no reaction activity. Although the conversion of aniline can be increased with dodecane and hexane, the selectivity of MA decreases sharply. When THF was employed, only 20% selectivity of MA was obtained at the 20% conversion of aniline. Ag catalyst on different supports (Al₂O₃, SiO₂, TiO₂) was tested. Al₂O₃ has suitable acidic and basic properties and interacts strongly with Ag, resulting in a superior performance when employed as support. In comparison, other metals supported on Al₂O₃ were also tested. Among them, Fe, Co, Ni, Cu, and Zn had no activity, while the main product over noble metals of Ru, Rh, and Pd was diphenylamine; only Ag could achieve good results. In this catalytic system, firstly, *COOH species are generated from CO₂ and H₂. Then, phenylcarbamic acid is obtained from aniline that reacts with the *COOH and subsequently dehydrates to formanilide, which is further dehydrated to get the target MA. Ag on Al₂O₃ primarily facilitates the heterolytic dissociation of H₂. The resulting H⁺ ions preferentially interact with the more basic MA, forming ammonium salts, which reduces the reactivity of MA and prevents further methylation to DMA ([Scheme 6]). However, in recycling experiments, the catalyst exhibited decreased activity due to Ag leaching, which reduced the Ag content [43].

Scheme 6 The reaction process for the N-methylation of aniline with CO₂ and H₂ over Ag/Al₂O₃. Reprinted with permission from Ref. [43]. Copyright 2023 Wiley. [rerif]

In 2018, Wang et al. designed a series of In₂O₃ nanocrystals with different density of grain boundaries. High density of grain boundaries (HGB-In₂O₃) exhibited excellent activity toward methylation of amines, the optimal yield of 82.7% for DMA with a mass activity of 21.2 mmol g⁻¹ h⁻¹ in N-methylation of MA was obtained. The presence of high density of grain boundaries not only facilitated the adsorption and activation of CO₂ to generate CH₃OH but also enhanced the activation of the N–H bond in amines ([Scheme 7]), which led to the attractive catalytic activity toward N-methylation of amines [44].

Scheme 7 The reaction process for the N-methylation of MA to DMA catalyzed by HGB-In₂O₃. Reprinted with permission from Ref. [44]. Copyright 2019. This is an open-access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). [rerif]

 

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Catalyst in N-methylation of amines with CO₂ and silane

In addition to H₂, some researchers have employed hydrosilanes as reducing agents in reactions. This is because the polar Si–H bonds in hydrosilanes are more prone to cleavage compared to the non-polar H–H bond in H₂, thereby generating activated hydrogen more readily, even without metal species. When H₂ is used as a reducing agent, reactions typically require high temperatures and pressures, whereas reactions involving hydrosilanes can proceed under milder conditions (low temperature and pressure). However, the price of hydrosilanes is high, and after the use of hydrosilanes, a byproduct containing silicon is formed, resulting in low atomic economy.

Non-metallic catalyst

In 2018, Han et al. introduced betaine groups (BE) onto the channel walls of pre-synthesized covalent organic frameworks (COFs), producing the [BE]X%-TD-COFs as the heterogeneous organocatalysts for the N-formylation and N-methylation of amines with CO₂ and phenylsilane (PhSiH₃). By controlling the reaction temperature and CO₂ pressure, the catalyst enabled the stepwise reduction of CO₂, selectively yielding different chemical products such as formamides and methylamines. With a lower amount of PhSiH₃ (2 equivalent), higher CO₂ pressure (5 bar), and lower reaction temperature (30 °C), the main products are formamides. With a higher amount of PhSiH₃ (4 equivalent), lower CO₂ pressure (1 bar), and higher reaction temperature (80 °C), the main products are methylamines due to the deep reduction of formamides [45]. In 2019, Liu’s group synthesized a biomass-derived nitrogen-doped porous carbon material (NPCs) through the pyrolysis of a mixture of tannic acid and urea. In NPCs, the pyridinic and pyrrolic nitrogen sites function as Lewis base centers, interacting with the carbon atoms in CO₂, weakening the C=O bond in CO₂, thereby lowering the activation barrier for CO₂ conversion and facilitating the subsequent methylation reaction. NPCs could catalyze the N-methylation of MA with CO₂ and PhSiH₃, achieving a 97.1% conversion of MA with a 90.2% selectivity to DMA [46]. In 2020, Dyson’s group developed a polymer ionic liquid catalyst, p[VBTAm]Cl. The excellent performance of p[VBTAm]Cl mainly arises from its unique structure, where chloride ions act as nucleophiles to activate PhSiH₃, forming a pentacoordinate silicon intermediate that facilitates nucleophilic attack by CO₂. At room temperature and under 1 bar CO₂ pressure, using PhSiH₃ as the reducing agent, N-formylation is the primary reaction. However, by increasing the pressure to 10 bar and raising the temperature to 100 °C, N-methylation becomes the predominant reaction [47]. In 2022, Hu’s group synthesized a novel poly-Lewis acid polymer compound, poly-BPh(C₆F₅)₂. The unique polymer structure endows it with exceptionally strong Lewis acidity, enabling efficient activation of CO₂ and promoting both N-formylation and N-methylation reactions of amines ([Scheme 8]). Under conditions of 30 °C, 1 bar CO₂, and 1 equivalent of PhSiH₃ as the reducing agent, the reaction with MA achieved a 98% yield of N-formylated product. When the temperature was raised to 50 °C and the amount of PhSiH₃ increased to 2 equivalents, a 96% yield of N-methylated product was obtained [48].

Scheme 8 Plausible catalytic cycle catalyzed by Poly-BPh(C₆F₅)₂ in N-methylation of MA with CO₂ and PhSiH₃. Reprinted with permission from Ref. [48]. Copyright 2022 Elsevier. [rerif]

 

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Supported metal catalyst

In 2015, Liu’s group developed an azo-MOP-Ru catalyst by coordinating Ru with an azo-functionalized microporous organic polymer (Azo-MOP). This microporous polymer exhibited a large surface area of 706 m²/g and a high CO₂ adsorption capacity (134.8 mg/g). Additionally, the azo ligand demonstrated excellent coordination ability with the ruthenium complex. Using the Azo-MOP-Ru catalyst, the N-methylation of MA to DMA achieved a yield of 99% under reaction conditions of 120 °C, 0.5 MPa CO₂, with PhSiH₃ as the reducing agent [49]. In 2016, the same group synthesized an azo-functionalized mesoporous polytriphenylphosphine (Poly(PPh₃)-azo) material. This material featured dual coordination sites, including triphenylphosphine and azo functional groups, which allowed it to coordinate with metal ions such as Ru³⁺ and distribute the metal uniformly across the porous support. The catalyst also achieved a 99% yield of DMA in the N-methylation of MA [50].

In 2019, Fei’s group introduced N-heterocyclic carbenes (NHC) into metal-organic frameworks (MOFs), leading to the development of the UiO-68-NHC catalyst. The NHC functional groups, through coordination interactions, effectively activate CO₂ and accelerate the reaction between hydrosilanes and CO₂, thereby significantly reducing the reaction energy barrier. Moreover, the metallized UiO-68-(NHC)ZnCl₂ and UiO-68-(NHC)CuCl catalysts efficiently protect the catalytic sites from decomposition by carboxylic acid functional groups, facilitating the catalytic conversion of amines containing carboxyl and ester groups. A good yield of 92% to DMA could be obtained in the N-methylation of MA under conditions of 60 °C, 1 bar CO₂, with the reducing agent PhSiH₃ in 2 h [51]. In 2020, Islam’s group immobilized Zn onto a covalent organic framework (TFP-DAQ COF), yielding the Zn(II)@TFP-DAQ COF catalyst. This COF material possesses a high specific surface area (1117 m²/g) and a mesoporous structure (pore sizes ranging from 3.5 to 35 nm), enabling efficient CO₂ adsorption and diffusion toward the catalytic sites. Zn(II)@TFP-DAQ COF catalyzed the N-methylation of MA; a remarkable yield of 98% to DMA could be obtained under reaction conditions of 80 °C, 1 bar CO₂, and 2 equivalents of polymethylhydrosiloxane (PMHS) [52]. The same group also supported Cu nanoparticles onto a triazine-based porous organic polymer (TzTa-POP), synthesizing the Cu-NPs@TzTa-POP catalyst. The Cu-NPs@TzTa-POP catalyst achieved a 91% yield of DMA in the N-methylation of MA [53]. Islam et al. also reported Ag-loaded porous organic polymer (POP) catalysts for the exclusive N-formylation and N-methylation of amines under ambient reaction conditions using CO₂ as a green C1 source. The porous polymer TTP-1 has been prepared with the aid of a straightforward Schiff base polycondensation reaction containing thiophene-2-carboxaldehyde and tetramine monomer [6,6′-(1,4-phenelyne) bis(1,3,5-triazine-2,4-diamine)], and PAN-T was synthesized through a Schiff base polycondensation reaction of melamine and thiophene-2-carboxaldehyde. By adjusting the solvent and temperature of the reaction, the successful production of formamides (DMF, MeOH, room temperature) and methylamines (CH₃CN, PMHS, 70 °C) could be accomplished with high levels of selectivity. POP-based materials Ag@TTP-1 and Ag@PAN-T are utilized for both the N-methylation and N-formylation of primary and secondary amines, and the product yields up to 98% could be achieved. Ag@TTP-1 demonstrated superior catalytic efficiency compared to Ag@PAN-T [54].

 

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Influence factor

Effect of support

In the context of CO₂ hydrogenation for N-formylation and N-methylation using heterogeneous catalysts, the support material plays a crucial role. A wide variety of supports have been reported in the literature, each exhibiting distinct functions. These support materials can be broadly classified into the following two categories: acidic/basic support and reducible support.

In heterogeneous catalysis for CO₂ hydrogenation leading to N-formylation and N-methylation reactions, a variety of acidic and basic supports, such as Al₂O₃ and Nb₂O₅, have been employed. This is due to the critical importance of the acid-base properties of the support. Acidic sites facilitate the adsorption and activation of amines, while basic sites enhance the adsorption and activation of CO₂ [38]. The synergistic interaction between these properties significantly improves both the reaction rate and selectivity of CO₂ hydrogenation. It was reported that the conversion of aniline and the selectivity to MA over Cu–ZrO _x /SBA-15 are much higher than that over Cu–ZrO _x /S-1 and Cu–ZrO _x /ZSM-5. The dispersion of Cu might not be the only factor affecting catalytic performance, and the acidity–basicity properties of the catalyst may also play a crucial role [31]. Au/Al₂O₃-VS was also more effective than Au/TiO₂, Au/ZrO₂, Au/CeO₂, Au/ZnO, and Au/SiO₂ [37]. Al₂O₃ with relatively strong acidic and basic sites exhibits the best catalytic performance. When TiO₂ and CeO₂ with a few acidic sites are used, the catalytic activity is moderate. Other supports, such as SiO₂ with few acidic sites, Mn₂O₃ and ZnO with weak acid–base properties, Co₃O₄ and C with neither acidic nor basic sites, and MgO with relatively strong basic sites, have little or very low catalytic activity [38]. For different supports (Al₂O₃, SiO₂, TiO₂) that supported Ag, Al₂O₃ has suitable acidic and basic properties and interacts strongly with Ag, resulting in a superior performance when employed as support [43]. For Au/MOF-808(Zr), H₂ is adsorbed and activated on the Au nanoparticles. CO₂ is adsorbed onto the acidic Zr⁴⁺ and basic O²⁻ sites, then reacts with activated H₂ to form formic acid for the N-methylation of amines [39].

Reducible supports, such as TiO₂ and In₂O₃, have also garnered significant attention in these reactions. This is because reducible supports tend to form stronger metal-support interactions and facilitate hydrogen spillover more effectively. Upon reduction, oxygen vacancies are generated, which not only serve as active sites for CO₂ adsorption but also enhance the activation of CO₂ molecules, thereby accelerating the overall reaction. For supported Cu catalysts, when CeO₂, ZnO, and C are used as supports, both the conversion and DMA selectivity are lower than when TiO₂ is used as the support [29]. However, the conversion of Cu loaded on TiO₂, CeO₂, and SiO₂ is lower than that of Cu loaded on In₂O₃ [30]. For supported Pt catalysts, TiO₂ as a support was much better than Al₂O₃, ZrO₂, SiO₂, and Nb₂O₅ when both Pt and MoO _x were loaded simultaneously [42].

 

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Metal

For the N-methylation reaction with CO₂ and H₂, metal species are usually required to activate H₂. However, the performance that metals can exhibit varies for different supports. Cu, Ag, Au, Pd, and Pt can exhibit excellent catalytic performance, especially in the presence of suitable modifiers. For example, He et al. studied In₂O₃-supported metal catalysts (Cu, Fe, Co, Ni, and Pd) in the N-methylation reaction of MA with CO₂ and H₂. Cu/In₂O₃ showed the highest conversion of N-methylaniline [30]. Wang et al. reported Au/Al₂O₃-VS was more effective than Pt/Al₂O₃, Ir/Al₂O₃, and Rh/Al₂O₃. A 92% yield of DMA in the N-methylation of aniline could be obtained over Au/Al₂O₃-VS [37]. Shimizu et al. reported a series of transition metal and MoO _x coloaded on TiO₂. Among various metals (Pt, Pd, Rh, Ru, Ni, Cu), Pt-MoO _x /TiO₂ showed the highest yield. The co-presence of Pt and Mo species on TiO₂ is critical [40]. Zhu et al. studied Al₂O₃-supported metal catalyst. Among them, Fe, Co, Ni, Cu, and Zn had no activity, while the main product of noble metals Ru, Rh, and Pd was diphenylamine; only Ag can achieve good results [43].

 

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Modifier

For PdGa/TiO₂, PdZn/TiO₂, and PdIn/TiO₂, during the preparation process of the catalyst, it undergoes hydrogen reduction to form PdGa, PdZn, and PdIn alloys. The formation of PdGa, PdZn, and PdIn alloy was favorable for the reduction of CO₂ to *HCOO and the following N-methylation reaction inhibited the C–N cleavage and the hydrogenation of the aromatic rings [34] [35] [36]. For supported Pt catalysts, Pt/TiO₂ exhibited the low conversion (7%) and yield (6%), but the catalytic performance was significantly improved after modification with oxides of transition metals (V, Mo, W, Re). Pt-MoO _x /TiO₂ exhibited the best performance (conversion 85%, yield 85%). The co-presence of Pt and Mo species on TiO₂ is critical [40]. There is no clear evidence for the role of MoO _x , and one possible reason is the acidity formed by MoO _x .

 

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Reaction conditions

The solvent has a significant impact on the N-methylation reaction. For example, Tomishige et al. reported that the conversion decreased from 22% to 4.4% as the amount of water added increased from 0 to 4 mmol in the N-methylation of aniline over Cu/CeO₂. The water produced during the N-methylation process as a byproduct would poison Cu/CeO₂ [28]. He et al. reported the N-methylation of MA to DMA over 1% Cu/In₂O₃. Among the tested solvents, octane is the best solvent; the conversion of MA is 73.4%, and the selectivity to DMA is 100%. Ether-based solvents, such as THF and DME, provide relatively low conversion, and almost no reaction occurs in alcohol since the oxygen in the solvent partially or completely blocks the existing oxygen vacancies, which may prevent the interaction between the substrate and the catalyst [30]. For Au/Al₂O₃ catalyst, Zhong’s group found that solvent polarity had a significant impact, apolar solvents showing better reactivity compared to polar ones [38]. For Pt-MoO _x /TiO₂ catalyst, Shimizu et al. found that the solvents of dodecane, water, and toluene gave moderate to good yield (45–72%), and methanol gave the lowest yield (34%). The reaction without solvent gave the highest yield of 85% [40]. Zhu studied the mono-N-methylation of aniline to N-methylaniline over Ag/Al₂O₃ in different solvents. When cyclohexane was used as a solvent, a 31% conversion of aniline with excellent selectivity (>99%) of the target product was achieved. Hexadecane, on the other hand, showed no reaction activity. Although the conversion of aniline can be increased with dodecane and hexane, the selectivity of N-methylaniline decreases sharply. When THF was employed, only 20% selectivity of N-methylaniline was obtained at the 20% conversion of aniline [43]. That is to say, non-polar straight chain alkanes have little interaction with catalysts and will not affect the adsorption and activation of CO₂ and subsequent N-methylation reaction.

In addition, temperature and pressure can also significantly affect the reaction. For example, Baiker’s group found that the higher the reaction temperatures, the higher the selectivity to methylamine in the N-methylation of NH₃ with CO₂ and H₂ over Cu/Al₂O₃ [23]. He et al. found that both the conversion and selectivity were enhanced progressively with the increase in temperature in the N-methylation of MA with CO₂ and H₂ over Cu/In₂O₃, probably because the higher temperature favors the activation of CO₂ and H₂ [30]. Tomishige et al. investigated the effect of CO₂ and H₂ pressures over Cu/CeO₂ in the N-methylation of aniline with CO₂ and H₂. They found that the CO₂ pressure had little effect on conversion and selectivity, while both conversion and selectivity increased with H₂ pressure [28]. Therefore, high temperature and high hydrogen pressure are favorable for the activation of CO₂ and its N-methylation reaction.

 

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The catalytic mechanism

CO₂ hydrogenation can form *HCOO, *CO, * CHO, * HCO, and other species [55]. Different metals, supports, and modifiers will form different species, which will affect the N-methylation reaction.

For Cu/TiO₂, the conversion of MA increased linearly with the amount of the exposed Cu⁺ and Cu⁰ species; the surface Cu⁺ and Cu⁰ species exposed on the catalysts were considered as the active sites. CO₂ is first hydrogenated to form the intermediate CHO*, which then reacts with MA to form MFA. Finally, MFA undergoes hydrogenation and dehydration to yield DMA [29].

For Cu/In₂O₃, the Cu⁺ was the main species on the surface of the catalysts, and it was considered the main active site. CO₂ was activated with H₂ to form formate, and then the formed formate was reacted with MA to form MFA; finally, MFA can be quickly hydrogenated to the final product of DMA [30]. For Cu–ZrO _x /ZSM-5, although it is believed that CO₂ was carbonylated with the adsorbed AN, generating phenylcarbamic acid, and it underwent hydrogenation to form formanilide, and further hydrogenation of formanilide formed MA [31]. However, the b-HCO₃* bands gradually weakened, the HCOO* bands gradually enhanced, and b-HCO₃* was gradually converted to HCOO* on the catalyst surface. Using HCOOH as a C1 resource resulted in an AN conversion of 18.4% and a MA selectivity of 47.3%, the conversion and selectivity with HCOOH are lower than that with CO₂/H₂ (~30%, ~95%, respectively); perhaps due to the small amount of HCOOH, it is easy to decompose at the reaction temperature. The possibility of *HCOO as an intermediate cannot be ruled out. For PdGa/TiO₂, PdZn/TiO₂, and PdIn/TiO₂, the formation of PdGa, PdZn, and PdIn alloy was favorable for the reduction of CO₂ to *HCOO and the following the N-methylation reaction [34] [35] [36]. The formation of H^δ– over PdIn alloy was favorable to the hydrogenation of CO₂ to *HCOO and MFA to DMA, inhibited the C-N cleavage, and inhibited the hydrogenation of the aromatic rings [36]. For Au/MOF-808(Zr), H₂ is adsorbed and activated on the Au nanoparticles. CO₂ is adsorbed onto the acidic Zr⁴⁺ and basic O²⁻ sites, and then reacts with activated H₂ to form formic acid. Amines adsorbed onto the CUS-Zr⁴⁺ acidic sites reacted with formic acid to produce N-formylated products. These N-formylated products subsequently react with activated H₂ to form N-methylated products [39]. For Pt-Mo _x /TiO₂, formic acid also formed by Pt-catalyzed hydrogenation of CO₂ [40].

For Ag/Al₂O₃, *COOH species are generated from CO₂ and H₂. Then, phenylcarbamic acid is obtained from aniline that reacts with the *COOH and subsequently dehydrates to formanilide, which is further dehydrated to get the target N-methylaniline. Ag on Al₂O₃ primarily facilitates the heterolytic dissociation of H₂. The resulting H⁺ ions preferentially interact with the more basic N-methylaniline, forming ammonium salts, which reduces the reactivity of MA and prevents further N-methylation to DMA [43].

For high density of grain boundaries catalyst of HGB-In₂O₃, the presence of a high density of grain boundaries facilitated the adsorption and activation of CO₂ to generate CH₃OH the followed the N-methylation of amines [44].

 

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Conclusions

Using of CO₂ as a C1 building block for the selective synthesis of bulk and value-added fine chemicals can become an indispensable part of a sustainable chemical industry. The key task of a large-scale application is to develop more effective CO₂ capture processes and discover and design powerful CO₂ reduction catalysts. Herein, we summarize the latest developments in N-methylation reaction of amines using CO₂ over heterogeneous catalysts. It reviews various types of heterogeneous catalysts, including noble metals (e.g., Pd, Ag, Au, Pt, Ru), non-noble metals (e.g., Cu, Zn), and non-metal catalysts (e.g., nitrogen-doped carbon materials). The catalytic performance, reaction mechanisms, and reaction conditions for achieving high selectivity and high efficiency in CO₂ hydrogenation and the following N-methylation are analyzed. This review offers insights that may inspire further research on these transformations.

Until recent years, heterogeneous catalysts based on precious and non-precious metals have achieved satisfactory yields of N-methylation products under relatively mild conditions. So far, the main drawback of all these developments has been the relatively low catalytic activity and selectivity. Therefore, further breakthroughs in cost-effectiveness methods are very eager. To significantly improve the reactivity of known catalytic systems and even design new catalysts, further understanding of their potential mechanisms will be crucial. There are competition issues between the N-methylation products of amines, such as cascade reactions, aromatic saturation, and C-N cleavage reactions. Therefore, CO₂ hydrogenation should be coordinated with N-methylation of amines. Given the thermodynamic stability and chemical inertness of CO₂, it is considered that the support needs to possess Lewis basic sites to adsorb and activate CO₂, while it also needs acidic sites to adsorb amines or create oxygen defects to activate CO₂. For the N-methylation with CO₂ and H₂, the activation mode of H₂ is usually not given much attention. Recent studies have shown that the activation of H₂ determines the activation pathway of CO₂ and the N-methylation reaction process. Therefore, more research should be conducted on the activation of CO₂ and N-methylation from the perspective of H₂ activation.

 

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Contributors’ Statement

Conception and design of the work: P.K. Luo, H.Y. Cheng; statistical analysis: Q.H. Gong, X.L.N. Su; drafting the manuscript: P.K. Luo; critical revision of the manuscript: H.Y. Cheng.

Conflict of Interest

The authors declare that they have no conflict of interest.

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Publikationsverlauf

Eingereicht: 31. Januar 2025

Angenommen nach Revision: 14. April 2025

Accepted Manuscript online:
24. April 2025

Artikel online veröffentlicht:
20. Mai 2025